1. Field of the Invention
The invention relates to a heat-resistant and corrosion-resistant article for transporting a hot, oxidizing gas having a surface to be exposed to the gas, which surface is made of an alloy having the following significant proportions by weight: chromium from 10% to 40%, if desired further elements including aluminum from 0% to 20%, silicon from 0% to 10%, at least one reactive element selected from the group consisting of yttrium, scandium and the rare earth elements, and the balance of at least one element selected from the group consisting of iron, cobalt and nickel.
The invention relates in particular to such an article for transporting a hot, oxidizing gas, where the gas has aggressive properties usually found in a waste gas in a gas turbine; in this context, the invention relates particularly to a structured part which serves to transport a waste gas in a gas turbine. This structural part can be a rotor blade or a guide vane, a heat shield or another highly thermally stressed part of a gas turbine. Particularly appropriate is such a part which in operation transports a waste gas having a mean temperature of more than 1000.degree. C., in particular between 1200.degree. C. and 1400.degree. C.
The alloy in the product is, in particular, an alloy of the type MCrAlY, where M is the major metal base of the alloy and is at least one element-selected from the group consisting of iron, cobalt and nickel, and where the alloy is further characterized by contents of chromium, aluminum or at least one element, selected from the group consisting of scandium, yttrium and the rare earth elements. The alloy of the type MCrAlY can also have contents of further elements, for example rhenium.
The invention also relates to a heat-resistant and corrosion-resistant article including a substrate of a nickel-based or cobalt-based superalloy, for example a gas turbine blade or another highly thermally and chemically stressed structural part of a gas turbine, which part has a protective layer including an alloy of the type mentioned.
Substrates for highly thermally and chemically stressed articles as are installed, in particular, in gas turbines are preferably made of superalloys based on nickel or cobalt by casting or forging, depending on the superalloy used. In casting such a substrate, it may be possible to resort to a technique which is known as "directional solidification" and gives a substrate having an anisotropic and/or spatially oriented microstructure, in particular a single-crystal microstructure. The superalloys which are generally suitable usually have excellent mechanical properties at the temperatures occurring during operation of the articles made therefrom. However, their chemical properties are sometimes such that they require particular measures for protection against corrosion. In order to offer this protection, specific protective layers for superalloys have been developed, in particular the protective layers including alloys of the type MCrAlY.
Typical protective layers of such a type are known from Published European Patent Application 0 412 397 A1. This document describes a protective layer having a high corrosion and oxidation resistance, which is characterized by it having a content of rhenium. Specifically, the protective layer has proportions by weight of the following elements: from 1% to 20% of rhenium, from 22% to 50% of chromium, from 0% to 15% of aluminum, where the proportions by-weight of chromium and aluminum together are at least 25% and at most 53%, from 0.3% to 2% of yttrium or an element equivalent to yttrium and also from 0% to 3% of silicon. The major metal base of the alloy forming the protective layer includes at least one of the elements iron, nickel and cobalt, plus impurities usually resulting from the manufacture in the usual proportions by weight. If desired, the protective layer can also contain proportions by weight of the following elements: hafnium up to 5%, tungsten up to 12%, manganese up to 10%, tantalum up to 5%, titanium up to 5%, niobium up to 4% and zirconium up to 2%. The sum of the proportions by weight of these elements should be at most 15%.
Further compositions for protective layers of alloys of the type MCrAlY are described in Published European Patent Application 0 532 150 A1, where nickel is in each case used as the major metal base of the alloy. Apart from the elements cobalt, chromium and aluminum which are always present, suitable additional elements are rhenium, hafnium, yttrium, silicon, zirconium, carbon and boron. In each case, the proportion by weight of aluminum in such an alloy is between 6% and 12%.
U.S. Pat. No. 4,451,299 describes protective layers of the type MCrAlY or MCrAlHf (Hf represents hafnium which can, under some circumstances, replace yttrium) having proportions by weight of aluminum between 7% and 20%. Nickel, cobalt and iron or mixtures of at least two of these elements are suitable as bases for the alloys described. In addition, proportions by weight of up to 10% of the elements platinum, rhenium, silicon, tantalum and manganese can be present. The protective layers produced from the alloys are said to be suitable for a temperature range between 650.degree. C. and 820.degree. C.
Published European Patent Application 0 207 874 A2 discloses a composition for an alloy containing the following proportions by weight: from 7.5% to 11% of aluminum, from 9% to 16% of chromium, from 2% to 8% of tantalum, from 0% to 25% of cobalt, and a basis which is essentially nickel. Such a protective layer applied to a substrate including an appropriately selected superalloy is said to have a particularly high diffusion stability. The diffusion stability is supposed to include the formation between the substrate and the protective layer applied thereto of only a small diffusion zone in which elements from the substrate mix with elements from the protective layer. As a result, at most an insignificant proportion of the aluminum diffuses from the protective layer into the substrate, which could cause the protective layer to lose the capability of forming a film of aluminum oxide on its surface which is essential to the oxidation resistance.
U.S. Pat. Nos. 4,321,310 and 4,321,311 each relate to a product in the form of a gas turbine component including a substrate of a superalloy; a metallic protective layer of the type MCrAlY applied thereto and, applied to the latter, a ceramic layer having a columnar crystalline structure, which functions as a thermal barrier layer. This thermal barrier layer makes it possible to increase the thermal stressability of the product, since the thermal barrier layer absorbs a high temperature difference and thus prevents the metallic particles of the product from being excessively stressed. The thermal barrier layer is bound to the product through a thin film of aluminum oxide which is formed by surface oxidation of the metallic protective layer. This surface oxidation can be carried out before or after application of the ceramic layer.
U.S. Pat. No. 5,262,245 describes an attempt to modify a superalloy for a product of the type in question in such a way that it is itself capable of forming a thin aluminum oxide film on its surface and thus makes the use of a metallic protective layer for anchoring a ceramic layer, as described above, superfluous.
U.S. Pat. No. 3,134,670 relates to corrosion-protected alloys which are formed mainly of iron, cobalt or nickel and are characterized by an addition of gallium. The alloys are supposed to be used for producing crowns, fillings and the like in dentistry, and also for producing household articles such as cutlery. The addition of gallium is said to improve the cutting and polishing performance of an alloy without impairing its hardness and toughness. The addition of gallium is also supposed to improve the castability of the alloy and to contribute to the formation of a fine-grained microstructure. There is no suggestion that an alloy described be used at particularly high temperatures.
The formation of a protective oxide film on the surface of the protective layer is an important function for an alloy in the context of a product of the type described in the introduction. Since such an oxide film steadily wears off during operation, it requires continual renewal. This renewal occurs through the use of continual oxidation of aluminum which diffuses from the protective layer to the surface, reacts there with oxygen and thus supplements the film. The maximum life of a protective layer is accordingly determined by its aluminum content, since with the loss of aluminum the protective layer loses the capability of forming the protective oxide film, and thus loses its protective action. Thus, a high aluminum content in an alloy for a protective layer is desired in order to give a long life.
However, a high aluminum content leads to embrittlement of the alloy. This is because the aluminum is not stored in the alloy in elemental form, but at least a significant proportion is present in the form of intermetallic compounds, in particular intermetallic compounds of nickel and aluminum or cobalt and aluminum. Accordingly, the aluminum content in an alloy to be used as a protective layer has to be restricted to a particular level. This level is determined by many factors and additions of elements such as rhenium can increase the maximum possible proportion by weight of aluminum in an alloy. As a general guideline, a proportion by weight of aluminum of 15% can be regarded as an upper limit. Such a proportion by weight does require very careful measures for matching the contents of the other elements in the protective layer so as to keep its brittleness within acceptable limits.
Problems similar to those for aluminum also occur in the case of the element silicon which is known as a constituent of protective layers and which likewise can develop a protective oxide film on an alloy. High additions of silicon also embrittle an alloy so that the proportion by weight of silicon also may not exceed a certain limit. The proportion by weight of silicon is therefore usually kept to less than 1% .